Myeloperoxidase (MPO), an important enzyme in the oxygen-dependent host defense system of human polymorphonuclear leukocytes, utilizes hydrogen peroxide to catalyze the production of hypochlorous acid, an oxidizing bactericidal agent. While MPO shows significant sequence homology with other peroxidases and this homology is particularly striking among the active site residues, MPO exhibits unusual spectral features and the unique ability to catalyze the oxidation of chloride ions. We have investigated the MPO active site with x-ray absorption and resonance Raman spectroscopies at neutral pH and also at the physiological acidic pH and have compared these results with those of horseradish peroxidase. The MPO heme environment was found to be inhomogeneous. At pH 7.5, the iron heme active site is 6-coordinate where the distal ligand is likely oxygen or nitrogen, but not sulfur. The heme is distorted compared to HRP, other peroxidases, and heme compounds, but at pH3, the distal ligand is lost and the heme is less distorted. HRP also loses its loosely associated distal water at this pH, but little change in heme distortion is observed. This change suggests that loss of the distal ligand in MPO releases stress on the heme which may facilitate binding of chloride ion.